In a current wireless communication system such as LTE (Long Term Evolution), a wireless base station and a mobile station communicate via a licensed frequency band. However, in the case of more data traffic being required such as watching videos online, a data transmission rate of the current wireless communication system is often not satisfactory. Therefore, in a future wireless communication system such as LTE-Advanced (LTE-A) subsequent to LTE, it is being studied that a License Assisted Access technology is used to further increase a throughput of a mobile communication system. In a cell employing the License Assisted Access technology, a base station and a user are allowed to communicate via an unlicensed frequency band used by other systems such as a WiFi system, in addition to the licensed frequency band.
Since the wireless communication system and other systems such as the WiFi system jointly use the unlicensed frequency band in the License Assisted Access technology, the base station and the mobile station need to listen to the unlicensed frequency band before transmitting data, to confirm that the unlicensed frequency band is available. In the WiFi system, an Access Point (AP) and a terminal device may listen to the unlicensed frequency band by using a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) listening strategy before transmitting data. In the strategy, the Access Point and the terminal device perform a random backoff algorithm by randomly selecting a backoff counter initial value (i.e. a number of times for backoff that need to be performed before transmitting data) according to a size of a contention window, where the size of the contention window may be adjusted according to an ACK/NACK signaling transmitted by the terminal device to the Access Point or an ACK/NACK signaling transmitted by the Access Point to the terminal device.